Conventionally cautions for the safe driving of a vehicle include a proper setting of an air pressure in a tire of the vehicle and attention to the wear of the tire. For example, a reduced air pressure in a tire increases the occurrence of flat tires and causes a burst at high speed, resulting in a serious accident. Therefore, drivers have to check tires on a daily basis.
However, even if tires are checked and preferred conditions are maintained for the tires, when a friction between a road surface and the tires decreases, for example, when a road surface is wet on a rainy day, skids occur under braking and move the vehicle in an unexpected direction, resulting in some accidents.
In order to prevent accidents caused by skids and a fast start, Anti-Lock Brake System (hereinafter referred to as ABS) and a traction control system are developed, and a stability control system having a YAW sensor is also developed in addition to these systems.
For example, ABS is a system of detecting a rotating state of each tire and controlling a braking force based on detection results so as to prevent the tires from locking.
The number of revolutions, an air pressure, a distortion, and so forth of each tire are detected as a rotating state of tires and detection results can be used for control.
For example, a detector for automatically detecting an air pressure of a tire is known. Such a detector uses a so-called “indirect” method of detecting an air pressure of a tire. In this method, data is inputted from an ABS speed sensor to estimate an air pressure of a tire.
As a method of detecting an air pressure of a tire that is used for the detector, the following methods are known: (a) a method of calculating a change (distortion) in the rolling radius of a tire by an angular speed of rotation of a wheel, the change being caused by a reduced air pressure, and (b) a method of performing FFT (Fast Fourier Transform) on an input signal and performing calculations using a change in the natural frequency of a tire.
On the other hand, as examples of a sensor and a tire that are used for the ABS, the traction control system, and the stability control system, U.S. Pat. No. 5,895,854 (hereinafter referred to as conventional example 1) and U.S. Pat. No. 6,308,758 (hereinafter referred to as conventional example 2) are known.
In conventional example 1, magnetic bar codes are provided which are arranged in lines in the circumferential direction of the side wall of a tire with adjacent parts alternately changed in polarity, and the bar codes are read by a sensor fixed on a chassis and a axis arm. Thus, the rotating speed of the tire can be detected. Further, the magnetic bar codes are provided in two or more lines in the radius direction of the tire, thereby calculating a force and deformation in the radius direction of the tire based on a phase difference between the detection results of the inner and outer magnetic bar codes.
In contrast to conventional example 1 having difficulty in forming magnetic bands at small intervals, conventional example 2 reduces the difficulty and discloses a tire improved in the resolution of magnetic bar codes arranged in lines in the circumferential direction of the side wall of the tire with adjacent parts alternately changed in polarity.
However, in conventional examples 1 and 2, since the magnetic bands are formed on the side wall when the tire is manufactured, it is quite troublesome to set the magnetic force of the magnetic band at the optimum value. That is, when the magnetic force of the magnetic band is much higher than the optimum value, a magnetic substance such as iron sand and an iron piece on a road surface may be adsorbed. Further, when the magnetic force of the magnetic band is lower than the optimum value, the detection of the sensor becomes difficult.
Moreover, the magnetic bands may gradually decrease in magnetization due to heat generated on the tire during the running of a vehicle and the detection of the sensor may become more difficult as running time increases.
In view of the above problems, an object of the present invention is to provide a tire distortion detecting method, a distortion detector, and a tire that can reduce the occurrence of a deterioration and can be used for a control system such as a stability control system.